New study reveals the powerful lure of plants

RECENTLY released research conducted by Professor Steve Johnson and his team over a period of six years reveals just how adept plants are at communicating with animals. Their study, published in Proceedings of the Royal Society: Biological Sciences, involved the plant Cytinus visseri. The study is the first to isolate the plant’s specific chemical scent and discover its effect on ground-dwelling mammals responsible for pollination.

It is well known that flowering plants use a variety of methods or signals to attract pollinators that facilitate their reproduction. Scent is one such method. It is particularly effective because of the diversity of volatile chemicals in flowers and because of animals’ heightened sense of smell. In addition, scent is extremely valuable to plants pollinated by small mammals that do their work at night when visual cues are less effective.

However, little research exists as to what kinds of scents are most attractive to pollinators. According to Johnson, humans find flowers that are pollinated by ground-dwelling mammals to be yeasty or pungent smelling. However, the chemical composition and attraction function of the floral scents of species pollinated by ground-dwelling mammals have remained unknown.

Cytinus visseri is a parasitic flowering plant that does not produce chlorophyll, but instead relies totally on its host plant. It produces dark maroon­, unusually robust flowers and resulting fruits that are hidden under the dense canopy of host shrubs. Its floral features are similar to those of other southern African plants which rely on pollination by ground-dwelling mammals.

The field component of Johnson’s study was conducted at Mauchsberg, the highest point on Long Tom Pass in Mpumalanga and home to the largest­ known population of Cytinus visseri plants. His colleagues included researchers from the South African National Biodiversity Institute, the University of Calgary and the German University of Bayreuth.

The researchers collected the scent from male and female flowers and analysed­ the samples in the labora-tory using gas chromatography mass spectrometry.

The scent comprised over 30 different compounds, with the main volatiles being 1-hexan-3-one, 3-hexanone and ethyl butyrate.

To ascertain the exact identity of the pollinators attracted to the scent, the researchers observed the plants for an extended period. Because of the nocturnal nature of some pollinators, the researchers placed cardboard discs covered with black soot around the plants to pick up the footprints of animals.

As expected, no insects or birds were observed. Instead, rodent footprints and tail-drag marks were found on the cardboard, clearly indicating that the pollinators were small nocturnal mammals — striped field mice, pygmy mice, dormice and elephant shrews.

To test the mammals’ response to the floral volatiles isolated in the laboratory, the team conducted trials with striped field mice by placing them in a y-shaped maze consisting of different scented pathways. The results revealed that one compound, 3-hexanone, consistently attracted the mice, suggesting that they find it innately attractive.

Previous research has revealed that 3-hexanone may also function in mammalian communication as it is present in the urine of some mice and has been found to elicit responses in the sensory centres of rats’ brains. Johnson suggests that the presence of 3-hexanone in Cytinus visseri plants may be the result of the plant adopting or taking on animal chemical scents during its process of evolution. Also, since the compound is indicative of certain foods, it may be effective at attracting small mammals. Johnson also noted that although 3-hexanone is not a component of most floral scents, it is found in some Central and South American plants that are pollinated by bats.

Interestingly, 3-hexanone has a pleasant aroma to humans which is borne out by the fact that it is used to flavour food, giving it a sweet grape or wine-like flavour.

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